A spectrophotometer includes a monochromator for obtaining monochromatic light of a preset or desired wavelength. The monochromator generally includes a light-dispersing element, such as a diffraction grating, and a driving mechanism for rotating the light-dispersing element. When the driving mechanism rotates the light-dispersing element, the orientation of the light-dispersing element with respect to the incident path changes, whereby wavelengths within a preset range are scanned. The wavelength range to be scanned is determined variously based on the purpose of the analysis, the type of spectrophotometer, and other factors. For example, the wavelength scanning range is preset at 190-1100 nm for an analysis of visible and ultraviolet light, and 700-1500 nm or 1000-2500 nm for an analysis of near infrared light. The angular range of rotation of the light-dispersing element, on the other hand, is preset at about 20-30 degrees, irrespective of the wavelength range.
Several types of mechanisms are used for rotating the light-dispersing element. One type includes a sine bar for converting a linear movement to a rotary movement, and another type uses an open loop control of a step motor and reduction gears. A third type using a closed loop control of a DC servomotor is also known.
With the first and second types of mechanisms, however, one cycle of the wavelength scanning takes a long time, often several tens of seconds. Another drawback of these two mechanisms that is known relates to wavelength discrepancy. For example, in all spectrophotometers, when the relation between the controlling inputs of the driving mechanism and the rotation angles produced thereby changes as a result of degeneration after a lapse of long time, a discrepancy arises between the wavelength desired by the user and the wavelength of the monochromatic light actually obtained. With the first and second mechanisms, however, the discrepancy cannot be detected immediately because they do not have a means for determining the rotational position of the light-dispersing element.
As for the third type of the mechanism using a closed loop control of a DC servomotor, the wavelength scanning speed is very high, and there is hardly any wavelength discrepancy because the mechanism includes a means for determining the rotational position of the light-dispersing element. This type of mechanism, however, has a drawback due to the use of the DC servomotor. That is, commercially available DC servomotors are designed for producing continuous rotation, and are not suitable for fractional rotation (for example, only 20 to 30 degrees out of 360 degrees), and are particularly unsuitable for producing reciprocal movement within a preset angular range. When this type of motor is used for rotating the light-dispersing element, it is difficult to stabilize the scanning speed within a preset angular range. Therefore, when output light from the light-dispersing element is sampled at a preset time point or points within a predetermined wavelength scanning range, the accuracy and reproducibility of the wavelength corresponding to each preset time point is poor.
In light of the above-described problems, an object of the present invention is to provide a monochromator constructed so that wavelength scanning can be carried out at a high and constant speed.